The present invention relates to a liquid crystal display device and more particularly to a driving circuit in the liquid crystal display device.
In the case of time multiplex driving of liquid crystal display elements, the average cross-talk voltage method is usually used as described in U.S. Pat. No. 3,976,362 to Kawakami and the polarity of voltages applied to liquid crystal layer is periodically reversed so that the liquid crystal layer has no mean DC level applied to it. For polarity reversal, there are two kinds of methods, one of which is to convert the driving waveforms into alternating waveforms by inverting the polarity within one frame period (the time necessasry to scan all scanning lines once), and is hereafter referred to as driving method A, and the other is to convert the driving waveforms by inverting the polarity within the period of two frames and is hereafter referred to as driving method B. These methods of time multiplex driving for liquid crystal display elements are discussed in detail, for example, in the Nikkei Electronics, Aug. 18th, 1980, pp 150-174.
The time multiplex driving for liquid crystal display elements is described in the above mentioned patent and reference, at present the driving method B is used mainly with the increase of scanning line numbers for time multiplexing in order to decrease the load of a driver LSI.
However, since the lowest driving frequency in the driving method B is the half of the frame frequency, e.g. 70 Hz, there may be the case that liquid crystal display elements are driven in very low frequency according to a pattern to be displayed. On the other hand, the threshold voltage of the liquid crystal has a characteristic dependent on frequency of applied voltages and in the case that the threshold voltage of the liquid crystal, a voltage at which ON-state of liquid crystal display elements begins to be visible, falls largely in lower frequencies, strong blurs occur in display according to particular display patterns when the driving method B is used. For example, if the liquid crystal has a characteristic in which the threshold voltage V.sub.th drops in lower frequencies as is shown in FIG. 1, and the alphabet E is displayed by applying voltage between signal electrodes C.sub.1,C.sub.2 . . . C.sub.20 and scanning electrodes R.sub.1,R.sub.2 . . . R.sub.27 selectively as in FIG. 2, the contrast of the shaded areas of A.sub.1,A.sub.2 and A.sub.3 is lower than that of the selected element D on B.sub.1 and B.sub.2 areas but higher than the non-selected areas E on B.sub.1 and B.sub.2. As a result, dark shades appear near an intended display as shadows. This phenomenon can be explained as follows. The frequency components of the driving voltage V.sub.0 applied to the liquid crystal display elements on the areas of A.sub.1,A.sub.2 and A.sub.3 are extremely lower than those of the driving voltage V.sub.0 applied to the liquid crystal display elements on the areas of B.sub.1 and B.sub.2. Considering the frequency dependence of the threshold voltage shown in FIG. 1, the voltage V.sub.1 applied to the elements on A.sub.1,A.sub.2 and A.sub.3 areas with respect to their threshold voltages at their frequency are higher than the voltage V.sub.2 applied to the elements on B.sub.1 and B.sub.2 areas with respect to their threshold voltages at their frequency and as a result, contrast of the elements on A.sub.1,A.sub.2 and A.sub.3 areas is higher than that of the non-selected elements on B.sub.1 and B.sub.2 areas and the phenomenon of blurs occurs around the display. As an example, the driving waveforms are shown in FIGS. 3 (a) to (j) which are applied to the display elements a.sub.1,a.sub.2,a.sub.3 and a.sub.4 shown in FIG. 2 by the driving method B. In these figure, by comparing the driving waveforms applied to the display elements A.sub.2 with the driving waveforms applied to the remaining display elements a.sub.1,a.sub.3 and a.sub.4, it can be understood that the frequency components of the driving waveforms applied to the display element a.sub.2 is extremely higher than the frequency components of the driving waveforms applied to the display elements a.sub.1,a.sub.3 and a.sub.4, and, from the relations shown in FIG. 1, it can be understood easily that the blurs in display become excessively conspicuous with the increase of frequency range of the driving waveforms. Further, in FIG. 2 the B.sub.1 area appears blanched compared with B.sub.2 areas due to the higher frequency components for the B.sub.1 area, and this phenomenon can be explained in the same way as above. Further, in FIG. 3 a symbol .tau..sub.d designates a pulse width of a scanning signal.
As a solution for this problem, it may be considered to use the driving method A, but it is known that different type of blurs in display appear by this driving method A.